'use strict';

function Snowflake(_workerId, _dataCenterId, _sequence) {
    this.twepoch = 1288834974657n;
    // this.twepoch = 0n;
    this.workerIdBits = 5n;
    this.dataCenterIdBits = 5n;
    this.maxWrokerId = -1n ^ (-1n << this.workerIdBits); // 值为：31
    this.maxDataCenterId = -1n ^ (-1n << this.dataCenterIdBits); // 值为：31
    this.sequenceBits = 12n;
    this.workerIdShift = this.sequenceBits; // 值为：12
    this.dataCenterIdShift = this.sequenceBits + this.workerIdBits; // 值为：17
    this.timestampLeftShift = this.sequenceBits + this.workerIdBits + this.dataCenterIdBits; // 值为：22
    this.sequenceMask = -1n ^ (-1n << this.sequenceBits); // 值为：4095
    this.lastTimestamp = -1n;
    //设置默认值,从环境变量取
    this.workerId = 1n;
    this.dataCenterId = 1n;
    this.sequence = 0n;
    if (this.workerId > this.maxWrokerId || this.workerId < 0) {
        throw new Error('_workerId must max than 0 and small than maxWrokerId-[' + this.maxWrokerId + ']');
    }
    if (this.dataCenterId > this.maxDataCenterId || this.dataCenterId < 0) {
        throw new Error('_dataCenterId must max than 0 and small than maxDataCenterId-[' + this.maxDataCenterId + ']');
    }

    this.workerId = BigInt(_workerId);
    this.dataCenterId = BigInt(_dataCenterId);
    this.sequence = BigInt(_sequence);
}

Snowflake.prototype.tilNextMillis = function(lastTimestamp) {
    var timestamp = this.timeGen();
    while (timestamp <= lastTimestamp) {
        timestamp = this.timeGen();
    }
    return BigInt(timestamp);
};
Snowflake.prototype.timeGen = function() {
    return BigInt(Date.now());
};
Snowflake.prototype.nextId = function() {
    var timestamp = this.timeGen();
    if (timestamp < this.lastTimestamp) {
        throw new Error('Clock moved backwards. Refusing to generate id for ' +
            (this.lastTimestamp - timestamp));
    }
    if (this.lastTimestamp === timestamp) {
        this.sequence = (this.sequence + 1n) & this.sequenceMask;
        if (this.sequence === 0n) {
            timestamp = this.tilNextMillis(this.lastTimestamp);
        }
    } else {
        this.sequence = 0n;
    }
    this.lastTimestamp = timestamp;
    return ((timestamp - this.twepoch) << this.timestampLeftShift) |
        (this.dataCenterId << this.dataCenterIdShift) |
        (this.workerId << this.workerIdShift) |
        this.sequence;
};

module.exports = Snowflake;
